1. depends on:
a) How the file is downloaded, if by web browser using HTTP protocol on port 80, then the web shield should scan it. If by other means then perhaps not during the download.
b) what the file type is, if executable of files that are at risk of infection, .exe, .jpg, .dll, etc. then when they are saved to the hard disk, they are newly created file and as such (if you haven't tweaked the default settings) the Standard Shield should scan it.
If using a download manager you can have it call the AV to scan (if it has this function). This is the file and path required, assumes that you installed avast in the default location
C:\Program Files\Alwil Software\Avast4\ashQuick.exe.
2. avast is a resident, on-access scanner and that is what n-access scanners do for files that are executable or are at risk of infection, there are exceptions; zip files for instance wouldn't be scanned as they are inert until you extract the file and point 1b above would apply.
On detection avast 4.8 will alert and stop it running awaiting your choice of action.
3. You don't really need any information as this is supposed to be a quick scan to enable you to get on with whatever it is you were doing without having to close any information window. Believe me if there is a virus you will know all about it (all hell breaks loose), as a visual and audible alert will happen. There is a setting where this can be changed if you feel it is required ?
4. No, the detections are technically the same if it can't find it in a normal scan, it is unlikely to find it in a boot-time scan, there are some exceptions.
The purpose of the boot-time scan is if having found a virus in normal mode but it can't be dealt with, file in use, etc. then a boot-time scan should get round that. So its purpose is more to do with scanning and being able to clean it when this is a problem when windows is fully running.